A whole of guys have asked me for a description of the nesting
fixture I use. So I'll try to describe it here.
I have a TriEx LM470D motorized crankup with positive pulldown.
This is one of those things that's a whole lot easier
to talk about when you can see it. But I don't think I have any digital
pictures of it. I think I have some of the prototype, which I made
out of wood. That's good enough to illustrate the point - so if I can
find the pictures I'll scan them in and send them to whoever is
interested.
The way it works is that each section comes down to rest on it's own
"platform" which then supports all of that section's weight. Each
platform is at the correct height so that all the telescoping sections
come to rest on their platforms at exactly the same time. The cable
winds up being equally slack on both the pull up and pull down
portions.
Each "platform" is comprised of two beefy aluminum beams mounted
vertically. One end of each beam rests on the concrete base and
the other end points upward. Each telescoping section comes down and
rests on top of a pair of such beams. The beam are held in place by
connections to the sides of the tower down low, below the point where
the telescoping travel. So the full length of each pair of beams
supports the weight of the section that rests on it. Even if one
beam were to fail - no tower sections would fall because there is
another beam still holding that tower section. Failure of one of
the beam is highly improbably since there is so little weight on
it compared to its strength. The beams are in longitudinal
compression and can support nearly 100 times the weight of the
heaviest LM470 telescoping tower section.
Even if you were to cut the cable, it wouldn't matter because
nothing would move. It's as safe as climbing a fixed tower at
that point.
The failure modes would be: 1) failure of two beams - unlikely with
less than 1000 pounds on something that will support more than
40000 pounds, or 2) The beams come loose from their mountings and
fall off to one side. However, it would be almost impossible for
this to happen and still have the tower retract seemingly normally.
And it is next to impossible for it to happen after the tower is
all the way down and you simply inspect it before climbing.
I only climb the tower when the telescoping sections have been lowered
onto their "platforms" and I look to make sure they are resting properly
and the cable is slack.
I apologize for being so poor at describing it.
I'll try to get some photos together over the next week or so and
anybody that requests them can have them and it will all make perfect
sense when you see it.
Michael WY6K
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